WATERTOWN — The shoot-out that crackled, ricocheted, and lit up the night in the middle-class neighborhood between the Armenian bakeries on Mount Auburn and the big-box stores of Arsenal Street was not a dozen rounds, or 20, as some terrified residents counted, or even 50.

It was a rattling, heart-pounding, incomprehensible 200 shots — in rapid-fire bursts and single shots, layered with shouting, punctuated with explosive booms, and interrupted by hold-your-breath pauses that never lasted long enough. When the firefight was over, an officer lay wounded, and one of the Boston Marathon bombing suspects was fatally injured
.

The other got away, escaping early Friday morning in a getaway that saw him drive over the body of his mortally wounded older brother even as officers emptied their guns into the car, authorities said.

Moments later, he would abandon the car and bolt into the darkness, setting off the final stage of an unprecedented manhunt that over 24 hours riveted the nation and become seared into local memory.

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The desperate and violent odyssey of 19-year-old Dzhokhar and 26-year-old Tamerlan Tsarnaev beganFriday night, 5½ hours after the FBI released the first images of the bombing suspects
.

Around 10:20 p.m. an MIT police officer, Sean Collier, 26, responded to a report of a disturbance near the MIT campus at Main and Vassar streets. Minutes later, police received reports of shots fired. When they arrived on the scene, they found Collier, a popular, hard-working patrolman, slumped in his cruiser with multiple gunshot wounds. Boston ­Police Commissioner Edward Davis called it an assassination.

Police almost immediately received a call about an armed robbery at a 7-Eleven a mile west in Cambridge. Authorities initially thought the brothers had robbed the store but later determined the crime was unrelated.

Two minutes after finding Collier, at 10:31, police received a call that two men had carjacked a vehicle at 816 Memorial Drive, near a Trader Joe’s plaza in Cambridgeport.

A few hundred yards up, a Mercedes sport utility vehicle had just pulled into the Shell at River Street, where two gas stations compete along Memorial Drive. From the Mobil across the way, a night clerk watched one man get out of the car and walk into the Shell station. Police would later identify him from surveillance tape as the younger of the two suspects. The carjacking victim got out and bolted across the street. He rounded the counter, ducked down and begged the night clerk to call 911, an employee said.

“I’m not sure if they released him or he went running for it,” said Martin El Koussa, a Mobil attendant who said his night-shift colleague described the scene to him.

The suspects fled the gas station in the black SUV, and several officers started hunting for the vehicle. When police found the SUV and chased after it, the brothers began throwing makeshift hand grenades and improvised explosives from the windows.

Davis said he had gone to sleep when he got a phone call from Superintendent-in-Chief Daniel Linskey, who said he was “in a car with lights and sirens and involved in a chase of the suspects in a running gun battle with police. He told me they were exchanging gunfire with the car and they were throwing explosives.”

The chase ended in a quiet neighborhood in Watertown. Police cornered the car and the shoot-out began.

Meanwhile, at the scene of the MIT shooting, officers had swarmed. Police dogs bayed, helicopters circled overhead. A State Police boat plied the Charles River, near the Longfellow Bridge. But when word came of the shoot-out, cruisers peeled away.

As they tore westward, they were joined by what seemed like every available police vehicle in Greater Boston, all whipping along Storrow and Memorial drives and lighting up the Charles in a seemingly unending blur of flashing lights.

In Watertown, as the shots pierced the stillness of the early morning, some residents were puzzled over the noises.

“I thought maybe people are doing fireworks, [so] I started heading toward the fireworks on my bike,” said Imran Saif, 28, a taxi driver who had been preparing to bike home to Cambridge after parking his cab on Watertown’s ­Melendy Street. “Then residents from windows just started shouting, ‘It’s gunshot fire! Go back the other way!’ ”

He described occasional big blasts around a succession of small sounds like “cherry bombs.” “It was just pop-pop-pop, pop-pop-pop,” said Saif, part of a growing crowd of media and neighbors who gathered on the edge of a rapidly extending police perimeter.

An MBTA police officer, Richard Donohue, was critically wounded and taken to the hospital.

The older bombing suspect, mortally wounded, was also hospitalized but doctors were unable to revive him.

After the gunfight and the younger brother’s disappearance, police vehicles raced around
and officers on foot barked commands. Davis described arriving at the scene.

“There were cars riddled with bullets,” Davis said. “There were shell casings all over the ground” and as he walked with officers he was cautioned to tread carefully among “exploded and unexploded devices on the ground.”

Now, law enforcement knew they had a dangerous suspect on the loose, who potentially had lethal explosives.

Authorities quickly set up a massive command post. Police vehicles raced frenetically, lights flashing, whipping around corners. Surrounding a home near Mount Auburn Street and Upland Road, a dozen or more officers suddenly dropped and drew their guns. “Get down, get down!” some shouted, while others pushed onlookers past the Giragosian Funeral Home. “Stay away! Backup, backup, backup!” But no shots could be heard.

Within minutes, onlookers gathered at the police tape stretched across Mount Auburn, also known as Route 16, trading stories of hearing gunfire or explosions.

Officers shouted at the crowds, “Make a hole! Make a hole!” for SWAT units and ­armored trucks heading into the cordoned-off search zone.

Some bystanders rooted on the officers.

“Get these guys, will ya?” a police scanner devotee from Newton said, lifting the tape with one hand to help an official SUV through. “Get em!”

On the other side of the tape marking the 20-street search zone, residents stayed low, whispering in darkened corners as packs of officers searched the streets outside.

“We got on the floor of the bedroom, we didn’t want to turn on any lights or make any noises,” said Cass Sapir, 34, who was awakened by gunshots at 12:50 a.m., saw police lights racing
near his home, and spent the next 18 hours laying low and following developments online with his wife.

By dawn, Governor Deval Patrick beseeched residents throughout the area to stay indoors while police searched homes and yards
.

As evening came, defeated-seeming officials held a press conference to say they had found nothing but it was safe for residents to come outside. That decree led a couple living just outside the search zone to spot a loose tarp on the boat in their yard. Looking inside they discovered the wounded Dzhokhar Tsarnaev.

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